GAIN WEIGHT really... is the easiest option.
Second, proper programming.
5/3/1 gives you a GREAT frame to work with. But for best results you need to create your own 5/3/1 template based on your own weaknesses.The stronger you are, the more important this is.
So you must an accurate estimate your own weaknesses first (This is the hardest part for most ppl) then you need to select the appropriate exercises to improve your weak points.
It was always strange to me why my strict OHP is so strong comparing to my bench (300lbs OHP, 400lbs bench) I found that Barbell always stick somewhere between 3/4 - 4/5 of the bench press.+ I never do any direct triceps work before, so it's a clear case - Lagging triceps.
So I cut a few set of chest/shoulder work here and there and I add 4-6s of heavy direct triceps work per every upper body day. And now im breaking all my rep PRs, and im preparing for new big 1rm :).
This is a clear case ofc, but if you focus on wrong muscle group, guess what...no progress.